1. Field of the Invention
This invention relates generally to geophones, and more particularly to suspension springs for geophones or seismometers.
2. Description of the Related Art
Geophones are devices which sense motion by suspending an inertial reference mass structure from a rigid, fixed supporting structure. Typically, the mass is a coil form suspended by springs in a magnetic field, one spring being attached at each end of the coil form. The springs position the coil form within the magnetic field so that the coil form is centered laterally and along its axis within the magnetic field. The springs also form a suspension system having a predetermined resonant frequency.
In seismic operations, seismic waves are imparted into the earth's crust at or near the earth's surface, and portions of those seismic waves are reflected or refracted from the boundaries of subsurface layers. Geophones are arranged in arrays or groups on the earth's surface, and when the reflected or refracted waves encounter a geophone, the coil form, which is suspended between the two springs, tends to stand still while the geophone housing and its connected magnetic circuit moves with the earth's surface. The movement of the coil form through a magnetic field causes a voltage to be generated at the output of the geophone. The outputs of the arrays of geophones are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape of subsurface formations, and the likelihood of finding an accumulation of minerals, such as oil and gas.
In present day geophones, spider springs are used extensively. Such springs are usually made from discs of spring material and have an inner ring and an outer ring which are connected by a plurality of legs. The legs are formed by etching or stamping the spring material in accordance with a predetermined pattern. Generally three such legs are used, and the three-legged arrangement is generally considered the most advantageous.
The legs of geophone springs generally have a rectangular cross-section and are curved along their lengths between the junctures with the inner and outer rings of the spring. After etching, the spring is "preformed" according to known techniques. When preforming is complete, the inner ring is offset or displaced relative to the outer ring, such that when a mass is suspended between two such springs, the inner ring, legs, and outer ring of each spring lie in the same plane.
A geophone is intended to sense motion from a direction which is roughly parallel to the axis of movement of the coil form with respect to the geophone housing. Therefore, it is desirable to eliminate or minimize the effects of any lateral motion of the coil form in response to forces which are not parallel to the axis of movement of the suspended coil form within the geophone.
In seismic operations, an impulse which is not truly parallel with the geophone axis and which contains a frequency component at or very near to that of the spurious frequency causes the geophone to produce an undesired or false EMF in the coils. Because of its high Q, the movement will continue for some time after the force which caused it has subsided. This resonance is considered to be the main spurious frequency in a geophone and it is highly undesirable as it limits the geophone's upper clean bandwidth.
Inasmuch as the geometry and the mass of the coil form for a given model of geophone are constant within manufacturing limits, the spurious resonance is also constant. The frequency of the spurious resonance can therefore be raised or lowered by changing the geometry of the suspension springs. This characteristic has been used to raise the frequency of these false signals until they are beyond the desirable frequency spectrum of the geophone by increasing the lateral stiffness of the spring. With this approach, these false signals do not interfere with or corrupt signals of interest. A common method of increasing the lateral stiffness of the spring is to shorten the spring legs. Unfortunately, the signal distortion caused by spring nonlinearity is increased when the legs are relatively short, and spring life is reduced.
It has been recently discovered that for a spring having a natural frequency of 10 hertz or less, a ratio of about 50 to 1 of the spurious resonant frequency to the natural frequency of the geophone can be achieved by straightening the legs as much as possible. Such springs exhibit a clean bandwidth at frequencies higher than previously thought possible. However, the useful life of such springs is shorter than other springs which exhibit less lateral stiffness and more lateral elasticity, because the stiffness of the legs does not allow the coil form to hit its normal intended stops for violent lateral movements. Accordingly, the art has sought to extend the useful life of laterally stiff geophone springs.
The art has sought an effective, simple device to give lateral elasticity to the two types of laterally stiff geophone springs: those having a high natural frequency, and those having a high ratio of the spurious resonant frequency to the natural frequency. Such lateral elasticity has been deemed desirable in order to withstand rapid acceleration or deceleration as encountered in hard usage or destruction tests of a geophone.